The rational design of biomedical prevention approaches to block HIV infection requires a better understanding of early events in HIV infection, particularly the viral and host factors that reduce acquisition and/or slow disease progression. Our research team is in a unique position to examine these questions during HIV infection because we have developed and maintained a cohort of ~ 2500 women at high-risk of HIV infection since 1993. This cohort includes >300 women who have known dates of HIV infection and who have been followed closely, including regular sample collection and clinical monitoring prior to and for many years after HIV infection. In addition, 21 women who have acquired a second HIV infection (superinfection) have been identified. Thus, in this cohort, susceptibility to infection can be detected as acquisition of a first HIV infection, acquisition of multiple variants from a single source partner and acquisition of a second infection. We propose to study HIV acquisition in this way because it allows us to gain the most comprehensive view of the biological factors that impact women's risk of HIV acquisition. Superinfection specifically offers the rare opportunity to examine immune mechanisms of protection by defining deficits in the immune responses in those who become superinfected compared to those with similar risk profiles who do not. Information on an immune correlate in this setting would provide critical insights for vaccine design and for monitoring of immune markers in vaccine trials, which currently is based primarily on information from model systems, not human studies. In the current grant, we propose to take advantage of this one of a kind, 20-year cohort study to address three aims that build on and leverage our recent findings. These include comprehensive studies of viral and host factors that impact the risk of acquisition as well as HIV disease progression in both singly and superinfected women.., We will also define deficits in HIV immunity that are associated with HIV superinfection, providing key insight into immune responses that are needed for an effective HIV vaccine.